Books are about feelings. We introduce the readers to certain characters and situations to make them feel a certain way. Different kinds of books give different feelings, which is good, because everyone has a slightly different taste in feelings. This is why different people like different genres.
One of my very favorite feelings to get from books is a boat feeling. I love those books where the plot happens in a confined space far, far away from civilization. I love the delicate balance of authority the captain has. I love the potential for mutiny, whether we're on the captain's side or whether he's the villain. I love the feeling that the entire world that matters is as small as a boat. So, when I randomly pulled a book off of the public library shelf--on the grounds that it had a dragon on the cover--and turned to the first page, I was delighted to find that it was a boat book. I checked it out and hurriedly finished the book I was currently reading so I could start this one. But after about twenty pages, I was much less interested. Where the book started as a boat book, it became quite apparent that it was not going to continue as such. As soon as the main character gets to land, he's going to start a new life training a newly hatched dragon.
I am now deeply disappointed. Not that there's anything wrong with dragon-training stories. Dragon-training stories have fantastic feelings. But if I'd known I was picking up a dragon-training story, I would have thought about it a little differently. And now I'm not sure whether or not I can trust this author, since she switched the story type on me like that. It's good, so I'll still read it. I'm just not so excited about it.
I was thinking about my sudden lack of interest in the book, and I realized that this has happened before. A book starts out as a boat book and a little while in, the characters get off the boat and start having the plot on land. And I always either stop reading or end up disappointed in some way.
When you start a story, you're setting a tone. The first page should give you the same kind of feelings that the book gives you overall. The first scene of a story should have something to do with the main theme. The first hundred pages should be consistent with the second hundred pages. We don't put corn flakes in cans and label them green beans. When you package your story as something it's not, your audience can't find you. If I had happened to hate boat books, but love dragon-training books, I might have put this one down after the first page.
If you start with an intense magical battle, don't give me a story about elementary school kids. If you spend half a book in an entirely realistic world, don't have the martians land or the neighbors turn out to be vampires. And if you're going start in the middle of the ocean with nothing between you and the waves but a few planks of wood, by all means, don't get off the boat.